The invention relates to position sensors. More specifically, the invention relates to an optical encoder for providing absolute position information.
Optical encoders are commonly used in printers to provide feedback information regarding rate and direction of rotation of a roller. When pressed against a sheet of paper, the roller feeds the sheet through the printer. The feedback information is used by a closed loop controller to maintain the roller at a constant rate of rotation and the sheet at a constant linear feed rate.
A typical optical encoder includes an encoder wheel having multiple encoder lines. Each line includes a spoke and a slot. The spoke and slot might be a reflective region and a non-reflective region, or an opaque region and a transparent region, etc. During operation of the optical encoder, a portion of the encoder wheel is illuminated. Encoder lines passing through the illuminated portion cast a shadow pattern onto a detector array. The shadow pattern includes a series of bands (e.g., dark regions and light regions). When the encoder wheel is rotated, the bands are moved across the detector array at a rate that is proportional to the rate of angular rotation of the encoder wheel. An encoder having a single channel (i.e., a one-channel encoder) could detect roller rotation rate for the controller. An encoder having a second channel (i.e., a two-channel encoder) could also detect direction of rotation for the controller.
In certain printers, however, it is desirable to know absolute position of the roller in addition to rate and direction of rotation. Such printers typically utilize three-channel encoders that provide the absolute position information. A three-channel encoder might include an additional reference slot in the encoder wheel and additional detectors in the array to detect the reference slot. Two of the three channels provide rotation rate and direction information, and the third channel is asserted whenever the encoder wheel is at zero degrees, thus providing absolution position information about the encoder wheel.
Adding the third channel increases the cost of the optical encoder. Part of the cost increase is due to the cost of the extra detectors used for the third channel. However, adding the third channel also adds mechanical contacts to the optical encoder and increases the size of the encoder wheel. The cost of extra material for the third channel can add a few dollars to the cost of the optical encoder.
Mass market products such as printers are extremely cost-sensitive. Any increase in the cost of the optical encoder will be absorbed primarily by the printer manufacturer. Increasing the cost of a single optical encoder by a few dollars might seem trivial, but it can have a substantial impact on the profits of a printer manufacturer, especially a printer manufacturer selling millions of printers per year.
There is a need for a low cost encoder that allows absolute position to be determined.